Estimates are higher for partitions of a table with @@use_stat_tables= PREFERABLY

set @@use_stat_tables=PREFERABLY;

CREATE TABLE t1 (i int, a VARCHAR(1000)  DEFAULT "AAA")

        PARTITION BY RANGE COLUMNS (a)(

    PARTITION p0 VALUES LESS THAN ('g') COMMENT "p0",

    PARTITION p1 VALUES LESS THAN ('m'),

    PARTITION p2 VALUES LESS THAN ('t'),

    PARTITION p3 VALUES LESS THAN ('w'));

ALTER TABLE t1 ADD PARTITION (PARTITION p4 VALUES LESS THAN (MAXVALUE));

INSERT INTO t1 VALUES (1,REPEAT('a',100)),(2,REPEAT('v',200)),(3,REPEAT('r',300)),(4,NULL);

INSERT INTO t1 VALUES (5,REPEAT('k',500)),(6,'April'),(7,7),(8,""),(9,"M"),(10,DEFAULT);

DELETE FROM t1 where a="";

DELETE FROM t1 where a=(REPEAT('a',100));

DELETE FROM t1 where a like "%v";

ALTER TABLE t1 ANALYZE PARTITION p1; 

MariaDB [test]> set @@use_stat_tables=PREFERABLY;

Query OK, 0 rows affected (0.000 sec)

MariaDB [test]> EXPLAIN PARTITIONS SELECT i,SUBSTRING(a,1,10) FROM t1 where (a>'m');

+------+-------------+-------+------------+------+---------------+------+---------+------+------+-------------+

| id   | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | Extra       |

+------+-------------+-------+------------+------+---------------+------+---------+------+------+-------------+

|    1 | SIMPLE      | t1    | p2,p3,p4   | ALL  | NULL          | NULL | NULL    | NULL |    7 | Using where |

+------+-------------+-------+------------+------+---------------+------+---------+------+------+-------------+

MariaDB [test]> set @@use_stat_tables=NEVER;

Query OK, 0 rows affected (0.000 sec)

MariaDB [test]> EXPLAIN PARTITIONS SELECT i,SUBSTRING(a,1,10) FROM t1 where (a>'m');

+------+-------------+-------+------------+------+---------------+------+---------+------+------+-------------+

| id   | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | Extra       |

+------+-------------+-------+------------+------+---------------+------+---------+------+------+-------------+

|    1 | SIMPLE      | t1    | p2,p3,p4   | ALL  | NULL          | NULL | NULL    | NULL |    4 | Using where |

+------+-------------+-------+------------+------+---------------+------+---------+------+------+-------------+

1 row in set (0.001 sec)